Conventional lead acid battery plates include a positive electrode (PbO.sub.2 plate) and a negative electrode (Pb plate) immersed in a sulfuric acid electrolyte and having a separator interposed therebetween. As a means of improving the ease of manufacture of such batteries, a variety of conductive additives have been proposed for incorporation into the plates. Lead dioxide has been proposed as an additive for paste mixtures containing tetrabasic lead sulfate, as described in Reich, U.S. Pat. No. 4,415,410, issued Nov. 15, 1983. Lead dioxide has also been formed in battery pastes by a reaction between lead monoxide and a persulfate salt (Reid, U.S. Pat. No. 2,159,226, issued May 23, 1939) or with ozone (Parker, U.S. Pat. No. 4,388,210, issued June 14, 1983, and Mahato et al., U.S. Pat. No. 4,656,706, issued Apr. 14, 1987). Lead dioxide enhances positive plate formation but provides no substantial advantages in the resulting battery because it participates in the positive plate reaction. During charging of the battery, lead sulfate is converted into lead dioxide, and the reverse reaction occurs during discharge.
Carbon has been used as a lead-acid paste additive, and has been used in combination with plastic materials in electrodes for bipolar lead-acid batteries, as described in Biddick, U.S. Pat. No. 4,098,967, issued July 4, 1978. Carbon, however, is not stable as a positive electrode material because it tends to oxidize. Thus, bipolar electrodes utilizing carbon as the conductive filler are not generally satisfactory for long-term use.
Metal oxides including titanium and tin oxides have also been suggested as additives for lead-acid battery plates. See, for example, Rowlette et al., U.S. Pat. No. 4,547,443, issued Oct. 15, 1985, and Hayfield, U.S. Pat. No. 4,422,917, issued Dec. 27, 1983. These additives have proven somewhat useful but fail to completely meet the need for a conductive additive which is economical, enhances plate formation and also improves the properties of the resulting lead-acid battery.
The present invention involves the use of conductive ceramics which exist in a perovskite crystal structure. The term "perovskites" as used herein refers to a large class of inorganic oxides which crystallize in a structure related to that of the mineral perovskite, CaTiO.sub.3. While the perovskite-type structure is ideally cubic, small distortions from cubic symmetry are common, as are superstructure variants in which a larger unit cell extended by a simple multiple in one or more dimensions is needed to account for some ordering of the cations among particular sites. Many oxides of perovskite-type structure are known to have minor departures in stoichiometry from the ideal formula ABO.sub.3. Thus, slight oxygen deficiencies up to about 5% are quite common.
Conductive metal oxides have been used in a variety of applications, for example, in polymeric compositions for electrical components as described in Penneck et al., U.S. Pat. No. 4,470,898, issued Sept. 11, 1984, and in corrosion-resistant coatings as described in Tada U.S. Pat. No. 4,352,899, issued Oct. 5, 1982. Superconductors resulting from solid solutions of BaPbO.sub.3 and BaBiO.sub.3 are also known. See Sleight, U.S. Pat. No. 3,932,315, issued Jan. 16, 1976, and Inagaki Japanese Patent Pub. 63-112423 (1988), which discloses compounds of the formula MPbO.sub.3, wherein M is Zn, Mn or Cd. Interest in such materials, however, has focused mainly on their superconductive properties.
Many patents describe the use of perovskite compounds such as barium metaplumbate (BaPbO.sub.3) in electrical components such as semiconductors, capacitors, and resistors. See, for example, Nitta et al., U.S. Pat. No. 3,374,185, issued Mar. 19, 1968, Hiremuth, U.S. Pat. No. 4,761,711, issued Aug. 2, 1988, Japanese Patent Pub. 63-136507, and Chemical Abstracts 106:147845v, 109:65406a, 85:185653m, 79:46649c and 79:46650w. Louzos, U.S. Pat. Nos. 3,898,100, issued Aug. 5, 1975 and 3,901,730, issued Aug. 26, 1975, describe the use of a wide variety of inorganic oxygen compounds of the formula X.sub.a Y.sub.b O.sub.c, generically including BaPbO.sub.3, for use in a cathode mix for solid electrolyte cells. Ruka U.S. Pat. No. 4,562,124 describes solid metal oxide solutions of perovskite-like structure for use in air electrodes for electrochemical cells. Otherwise, little attempt has been made to utilize perovskite compounds in battery electrodes.
Some types of perovskite materials have also been generally proposed for use as electrode coatings for electrodes used in electrolytic processes. See, for example, Peterson, U.S. Pat. No. 4,032,427, issued June 28, 1977, describing a family of oxide bronze compounds, and Kudo et al. U.S. Pat. No. 3,861,961, issued Jan. 21, 1975. However, many perovskites are not stable in sulfuric acid, and there remains a need for an electrode which is suitable for use in electrolytic processes conducted in sulfuric acid solutions, and which can be simply and inexpensively prepared.
The present invention further concerns electrodes for use in electrochemical processes. Many known electrolytic processes are conducted in a sulfuric acid environment, for example, processes for the synthesis of ozone, manganese dioxide, acetylene dicarboxylic acid, adiponitrile, tetramethyl lead, hexahydrocarbazole, .alpha.-methyldihydroindole, dihydrophthalic acid, and anthraquinone. The materials used to make electrodes for these processes have various drawbacks, such as expense and poor performance characteristics. For example, in the conventional synthesis of electrolytic manganese dioxide (EMD), the anode substrate used is either carbon, lead or titanium, with or without surface treatment. Each of these materials has limitations when used in such an anode. The quality of MnO.sub.2 deposited on a carbon anode is usually not as good as that from a titanium anode. A lead anode is too soft, and lead impurities are detrimental to the performance of the manganese dioxide product. A titanium anode is sufficiently strong, but has problems with dissolution into sulfuric acid, passivation during deposition, and high cost. To overcome the passivation problem, sand blasting or coatings such as .beta.-MnO.sub.2, RuO.sub.2 are used. As an additional disadvantage, the Ti anode surface must be treated after only a few cycles of deposition and stripping.
The reaction for production of ozone from water at an anode is: EQU 3H.sub.2 O=O.sub.3 +6H.sup.+ +6 e.sup.- E.sup.o =+1.6 V (1)
An oxygen evolution reaction competes with the ozone reaction: EQU 2H.sub.2 O=O.sub.2 +4H.sup.+ +4e.sup.- E.sup.o =+1.23 V (2)
At the cathode either of two reactions may be selected, namely hydrogen evolution: EQU 2H.sup.+ +2e.sup.- =H.sub.2 E.sup.o =0.0 V (3)
or oxygen reduction: EQU O.sub.2 4H.sup.+ +4e.sup.- =2H.sub.2 O E.sup.o =-1.23 V (4)
Several electrolytic processes for producing ozone have been described in the art. In one such process, 03 is evolved into a stream of water from the back of a porous PbO.sub.2 anode in contact with a solid polymer (perfluorinated sulfonic, Nafion.RTM., membrane) electrolyte (reaction 1). Hydrogen evolution (reaction 3) is the cathodic reaction. See Stucki et al., Abstract No. 573, The Electrochemical Society, Extended Abstracts, Vol. 83-1, p. 866, San Francisco, May, 1983. The advantages of this approach are that high current densities are obtained, the PbO.sub.2 electrode is more stable with the solid polymer than with an acid electrolyte, the ozone is dissolved directly in the stream of water and used for water treatment, avoiding the hazards of gaseous ozone, and the reaction can be run at room temperature. A disadvantage is the high voltage of the reaction, which increases the power consumption.
According to another known process, O.sub.3 is evolved as a gas at a glassy carbon electrode in tetrafluoroboric acid (reaction 1). Lead dioxide cannot be used as an electrode material because it is not stable in tetrafluoroboric acid. The cathodic reaction is oxygen reduction (reaction 4). See generally, Foller et al., Ozone: Science and Engineering, Vol. 6, pp. 29-36, 1984, and Foller et al., U.S. Pat. Nos. 4,316,782, issued Feb. 23, 1982 and 4,541,989, issued Sept. 17, 1985. A stream of air is passed over the air electrode. Advantages of this approach are that power consumption is lower due to the lower overall cell voltage with oxygen reduction compared to hydrogen evolution, there is no need to add water to maintain the material balance of the cell, since oxygen is reduced to form water, and hydrogen, which can be hazardous, is not evolved. Disadvantages are that the maximum current density of an air cathode is low (about 250 mA/cm.sup.2), the anode must be cooled to about 0.degree. C. (this adds complexity and cost), and the ozone gas must be immediately diluted with O.sub.2 to prevent explosions. The diluted ozone/oxygen stream also remains hazardous.
Other electrolytes which have been used to produce ozone are sulfuric acid, phosphoric acid, and various electrolytes containing fluoride ions. The advantage of the fluoride ions is that they reduce the rate of competing oxygen evolution (reaction 2) and hence increase the current efficiency of ozone production.
Foller and Tobias (See J. Electrochem. Soc., Vol. 129, pp. 506-515 and 567-570, 1982) and Kotz and Stucki (J. Electroanal. Chem., Vol. 228, pp. 407-415, 1987) have shown that ozone can be produced at lead dioxide anodes in sulfuric acid solutions. The efficiency of the process is low because oxygen evolution (reaction 2) competes with ozone production. Another problem is that the lead dioxide is not stable. Kotz and Stucki concluded that there is a need for a conductive anode material with a high overpotential for the oxygen evolution reaction.
The present invention provides electrodes used in lead-acid batteries and in electrolytic processes which can meet the foregoing needs.